A gas diffusion electrode (GDE) consumes or is depolarized by a gas feed while allowing direct electronic transfer between the solid and gas phase. Together with the electrolyte, the GDE provides a path for ionic transfer, which is just as critical. GDEs are typically constructed from a conductive support, such as a metal mesh, carbon cloth, or carbon paper. This support is often called a web. The web is coated with hydrophobic wet-proofing layers, and finally, a catalytic layer is applied most commonly to one face. While the catalytic layer can consist of very fine particles of a precious metal mixed with a binder, many employ the methods similar to that in Petrow, et al., U.S. Pat. No. 4,082,699. This patent teaches the use of using finely divided carbon particles such as carbon black as the substrate for small (tens of angstroms) particles of the nobel metal. Thus called a "supported" catalyst, this methodology has shown superior performance and utilization of the catalyst in electrochemical applications. However, the application of this supported catalyst as well as wet proofing layers to the web engages the need for a well-dispersed mix.
Often, GDEs are cited as key components in Fuel Cells. Here, the anode is typically depolarized with hydrogen while the cathode is depolarized with oxygen or air. The resulting products are energy in the form of electricity, some heat, and water. Examples of acid or alkaline fuel cells are well known. However, some have also realized that the energy-producing quality of a fuel cell can be adapted to industrial electrochemical processes and thus save energy and hence reduce operating costs.
GDEs also may allow the creation of a commodity directly from a gaseous feedstock. For example, Foller, et al. (The Fifth International Forum on Electrolysis in the Chemical Industry, Nov. 10-14, 1991, Fort Lauderdale, Fla., Sponsored by the Electrosynthesis Co., Inc.) describe the use of a GDE to create a 5 wt. % hydrogen peroxide in caustic. In this case, oxygen is the feedstock and a specific carbon black (without noble metals) is the catalyst. A typical chlor-alkali cell uses two solid electrodes to produce sodium hydroxide and chlorine. In this case, both the anode and cathode expend energy to evolve gas, and special measures are taken to keep the resulting hydrogen away from the chlorine due to a potentially explosive mixture. The typical chlor-alkali cathode can be replaced with an oxygen-depolarized cathode, as has been shown by Miles et al. in U.S. Pat. No. 4,578,159 and others. A cell run in such a manner saves approximately one volt, and the hydrogen/chlorine problem is eliminated. Aqueous hydrochloric acid is an abundant chemical by-product. One can recover the high-value chlorine by oxidizing solutions of HCl, and thus recycle the chlorine as a feedstock to the chemical plant.
Electrolysis becomes extremely attractive when the standard hydrogen-evolving cathode is substituted with an oxygen-consuming gas diffusion electrode due to the significant drop in energy consumption. The ability of the gas diffusion electrode to operate successfully in this and the preceding examples is acutely dependent on the structure of the gas diffusion electrode: for in all these cases, the electrode serves as a zone for liquid-gas-solid contact, as a current distributor, and most importantly, as a liquid barrier. The use of solid polymer electrolytes has greatly expanded the field of electrochemistry. As summarized above, electrochemical processes depend on the transfer of ionic and electronic charge through the use of an anode, cathode, and an ionic liquid electrolyte. However, with the advent of the solid polymer electrolyte fuel cell, the traditional liquid phase has been replaced with a membrane composed of a polymer electrolyte that transfers ionic charge under typical electrolytic conditions. One can deposit a catalyst layer directly on the membrane, or attach a gas diffusion electrode to one or both faces of the conducting membrane. Such an assembly can be called a membrane electrode assembly (MEA), or for fuel cell applications, a PEMFC (proton exchange membrane fuel cell).
These solid polymer electrolytes are often composed of ion-conducting membranes that are commercially available. For example, in addition to the previously mentioned Nafion (a cation exchange membrane), Asahi Chemical and Asahi Glass make perfluorinated cation exchange membranes whereby the ion exchange group(s) are carboxylic acid/sulfonic acid or carboxylic acid. These companies produce cation exchange membranes with only the immobilized sulfonic acid group as well. Non-perfluorinated ion exchange membranes are available through Raipore (Hauppauge, N.Y.) and other distributors such as The Electrosynthesis Co., Inc. (Lancaster, N.Y.). Anion exchange membranes typically employ a quaternary amine on a polymeric support and are commercially available as well.
Nafion is typically employed in some fuel cells. For the hydrogen/air(O.sub.2) fuel cell, hydrogen and oxygen are fed directly to the anode and cathode respectively, and electricity is generated. For these "gas breathing" electrodes to perform, the gas diffusion electrode structure must be highly porous to allow three phase contact between the solid electrode, the gaseous reactant, and the liquid or near liquid electrolyte. In addition to providing a zone for three-phase contact, the gas diffusion electrode structure aids in making electrical contact to the catalyst, enhances transport of reactant gasses into the zone, and provides for facile transport of product away from the zone (e.g. water vapor).
In addition to a gaseous hydrogen fuel and gaseous air (O.sub.2) oxidant, others employ a mixed phase system such as the methanol/air(O.sub.2) fuel cell. Here, liquid methanol is oxidized at the anode while oxygen is reduced at the cathode. Another utilization for ion-conducting membranes and gas diffusion electrodes includes the electrochemical generation of pure gasses [for example see Fujita et al. in Journal of Applied Electrochemistry, vol. 16, page 935, (1986), electro-organic systhesis [for example see Fedkiw et al. in Journal of the Electrochemical Society, vol. 137, no. 5, page 1451 (1990)], or as transducers in gas sensors [for example see Mayo et al. in Analytical Chimica Acta, vol. 310, page 139, (1995)].
Typically, these electrode/ion-conducting membrane systems are constructed by forcing the electrode against the ion conducting membrane. U.S. Pat. No. 4,272,353; No. 3,134,697; and No. 4,364,813 all disclose mechanical methods of holding electrodes against the conducting membrane. However, the effectiveness of a mechanical method for intimately contacting the electrode to the polymer membrane electrolyte may be limited since the conducting membrane can frequently change dimensions with alterations in hydration and temperature. Swelling or shrinking can alter the degree of mechanical contact.
Thus, an alternative method of contacting the electrodes with the polymer membrane electrolyte involves direct deposition of a thin electrode onto one or both sides of the conducting polymer substrate. Nagel et al. in U.S. Pat. No. 4,326,930 disclose a method for electrochemically depositing platinum onto Nafion. Others have employed chemical methods whereby a metal salt is reduced within the polymer membrane [for example see Fedkiw et al. in Journal of the Electrochemical Society, vol. 139, no. 1, page 15 (1192)].
In both the chemical and electrochemical methods, one essentially precipitates the metal onto the ion conducting membrane. This precipitation can be difficult to control due to the nature of the ion-conducting polymer membrane, the form of the metal salt, and the specific method employed to precipitate the metal. As the goal of a thin, porous, and uniform metal layer is often not met via precipitation, practitioners have turned to other deposition methods. For example, ion beam assisted deposition techniques are disclosed in co-pending provisional patent application by Allen et al. (Ser. No. 60/035,999); a method for coating the membrane with an ink composed of the supported catalyst and solvent is disclosed by Wilson and Gottesfeld in the Journal of the Electrochemical Society, volume 139, page L28, 1992; and a method of using a decal to deposit a thin layer of catalyst onto the ion-conducting membrane is summarized by Wilson et al. in Electrochimica Acta, volume 40, page 355, 1995. Thus, these approaches differ from the previous strategy by the catalyst layer being deposited onto the ion conducting membrane, and a gas diffusion structure is subsequently placed against this catalyst layer.
Regardless of whether the catalyst is fixed to the membrane, or coated onto an uncatalyzed gas diffusion electrode and then bonded to the membrane via mechenical, and/or thermal means, the structure and composition of the component contacting the catalyst contributes to the overall MEA performance. This component is variously called a "diffuser", an electrode "backing", "gas diffusion media", a "gas diffusion layer", or an "uncatalyzed gas diffusion electrode" and can predominate MEA performance during operation at high current density. We will use the term diffuser to encompass all these synomyms. A diffuser is a material that: 1) provides electrical contact between the catalyst and the electrochemical cell current collector, 2) distributes and facilitates efficient transport of feed gas or gasses to the electrode, and 3) becomes a conduit for rapid transport of product(s) from the electrode. Thus the electrode is the catalytic layer or zone characterized by a three-phase interface of solid, liquid, and gas whereas the diffuser is a two-phase structure for gaseous (or liquid) transport and electrical contact.
There are a few commercial providers for diffusers. Gore Associates (Elkton, Md.) offer Carbel.TM., a conductive, microporous polymer. E-TEK, Inc. (Natick, Mass.) offers uncatalyzed versions of the gas diffusion electrodes found in their catalog. Of these, the uncatalyzed ELAT.TM. is listed as the best material for MEA applications. The gas diffusion electrode structure designed for providing a three-phase zone, current contact, and a liquid barrier is being adopted for MEA applications.
Typical ELAT construction is detailed in U.S. Pat. No. 4,293,396 by Allen et al.. Here, a carbon cloth serves as the web. Carbon black is prepared for application to the carbon web by using techniques listed in U.S. Pat. No. 4,166,143 whereby solutions of Vulcan XC-72 or Shawinigan acetylene black (SAB) are mixed with water, ultrasonically dispersed with a sonic horn, mixed with Teflon.RTM. (TFE), and filtered. Layers of SAB mix serve as the wetproofing layer on each side of the web. Finally, layers of (catalyzed) Vulcan mix are coated onto one side of the assembly. Although the importance of mix penetration into the web is discussed, the actual coating method is not disclosed. The reported products were of limited lot size, so may have been individually prepared. After the final coat, the assembly may be sintered in air at a temperature sufficient to cause the Teflon to flow, typically 300-350.degree. C. This double sided structure was designed with the intent to create an electrode that both achieves good gas distribution and contact with the catalyst while providing a hydrophobic barrier to prevent electrolyte transport completely through the electrode. Regardless, no information is relayed as to how this structure could be produced with economical means.
Similarly, a typical ink application is described by Ralph et al. in The Journal of the Electrochemical Society, Vol. 144, page 3845, 1997 and references therein. Here, the goal is to minimize platinum usage. A gas diffusion electrode is constructed by using silk screen technology to coat a carbon paper web. The ink is comprised of catalyzed carbon black and binders including Teflon. The authors claim a resulting electrode structure comparable to that described by wilson and Gottesfeld or Wilson et al. cited previously above. If GDEs and ion conducting membranes are to be used in large volume, commercial processes such as power generation in electric vehicles, then one must meet a significant reduction in component cost. Thus, while the authors endorse the need for inexpensive manufacturing processes, they describe a batch coating design, which inherently limits product throughput.
In both the ink and mix preparation methods, it is generally accepted that the ultrasonic horn serves an important role in dispersing the carbon in solvent. Since the carbons are high surface area substances, it is important to prepare a uniform and stable suspension. Carbon blacks do not "wet-out" without a significant input of energy or shear into the solution. Some also modify the solution with additives as well to induce high shear. The ultrasonic horn performs this function of wetting-out by way of high frequency electrical energy directed from a stainless steal tip immersed in the solution. The action of the horn generates pressure waves through the vessel and produces high shear through cavitation. Although suitable for limited production runs or R&D sized samples, there are several limitations to ultrasound. First, since the energy is projected from a single source, i.e., the horn, the power is a function of the distance from the horn, and will diminish significantly as one moves away. Second, as the action of the carbon black on the horn leads to abrasion and accelerated corrosion, the projected power spectrum emanating from the horn changes in time. For these reasons, ultrasound may not be appropriate for production of large quantities of diffusers.
With the rise of PEMFCs as suitable clean power sources, and the parallel increase in the use of MEAs in industrial and sensor applications, there is a need for a diffuser tailored for these materials. The current diffuser technology employs structures that were originally designed for liquid electrolyte systems. In addition, the current routine use of the sonic horn produces carbon black dispersions for coating that may be non-uniform and difficult to control for production of large batches of diffuser. Furthermore, the current manufacturing methodology is limited in its applicability to continuously coating a web--a step believed to be crucial in producing an inexpensive product.